In my ZS captive portal I authenticate with multiple external AD. It works fine.
I can choose easily the realm in the list. But is it possible to show an alias instead of the realm in the authenticating forms ?
No, it is not possible. Active Directory uses Kerberos5 as main authentication method and the captive portal of Zeroshell needs to authenticate the users against the KDC of AD by specifing the principal in the form user@REALM.